1. Field of Invention
This invention relates to foot pedal force transducers, and more particularly to foot pedal force transducer apparatus for measuring the force applied by an operator to the brake pedal of an automobile.
2. Description of the Prior Art
Automobile diagnostic testing has significantly increased in recent years due to the interest in extending the useful life of an automobile and also due to the availability of computerized diagnostic equipment, such as AUTOSENSE.RTM.. These diagnostic testing procedures have been adopted both by small repair garages and also by the major motor vehicle manufacturers for production testing of major systems within the vehicles to improve overall quality control and to reduce the infant mortality rate of failures inherent in new products built on large volume production lines. Similarly, the laws in many states require periodic motor vehicle inspection of safety related functions which include exhaust emissions, proper lighting, and brake systems, among the many functions which may be tested. With respect to automobile brake system testing the most practical method is an in-use type of test which tests the ability of the braking system to stop the car under some form of actual movement conditions. This may vary between actual acceleration and braking of the automobile over a short distance runway of some sort, as commonly used in state motor vehicle inspection procedures, to the use of dynamometer test stands which allow stationary acceleration of the automobile drive train to permit dynamic testing of both engine performance, exhaust emission and braking ability. Dynamometer test stands may be more prevalent among the auto manufacturers, but have found increased use among the smaller repair facilities.
The type of brake testing described here involves the use of some type of sensor which measures the force (in pounds) applied to the brake pedal of the automobile under test simultaneously with measurement of the decreased velocity and stopping time of the drive train, as may be evidenced by the measurement of the decreasing revolutions of the vehicle drive wheels on the dynamometer. The vehicle under test must be accessible to the testing apparatus, or test console and the various sensors and handheld units used with the test console, all of which are connected through umbilical cords which house the electrical interconnection between the respective devices and the test console. This is generally provided by having the sensors suspended by the umbilical cords in some sort of overhead track adjacent to the test console and dynamometer test stand. The operator performing the test from inside the automobile selects the particular testing device and draws it by its umbilical cord into the car. There is significant human engineering involved in providing the most practical design for these operator used devices to minimize the possibility of human error in performing the testing. A commonly used test stand diagnostic tool, in addition to the brake sensor, is a handheld keyboard which allows for communication between the operator in the car and the test console while the car is in dynamic test on the dynamometer. Although there has been significant emphasis on providing the most efficient design for the handheld keyboards to permit easy access by the operator in and out of the automobile and to simplify as much as possible the operating procedures such that the units may be used by semi-skilled persons, little has been provided in the way of simplification of sensor equipment for measuring the automobile braking system.
Typical of the presently used brake testing sensors is a handheld foot pedal force transducer having a hollow shaft housing, approximately two feet long, which includes a base plate rigidly attached at a right angle to the bottom of the shaft. A movable faceplate is hinged at the furthest extension of the base plate from the shaft and is connected at the other end to a spring mechanism inside the shaft housing. The spring mechanism includes a main spring having a defined spring constant (K) and one or more balancing springs to provide a quiescent position for the movable face-place when no force is applied to the plate. In using this device the operator must hold the transducer such that the base plate is aligned in position on the face of the brake pedal, and then apply braking force with his foot to the movable faceplate causing it to compress against the main spring to some position which allows transmission of the applied foot force through the base plate to the brake pedal. The magnitude of the force applied is proportional to the spring constant and the travel distance of the spring. The spring is attached to the wiper of a potentiometer and the force magnitude is read out as a voltage signal at a magnitude proportional to that of the applied force. This device has a number of disadvantages including inaccuracy of force measurement and limited practical utility. Although this type of transducer has been in use for many years the inherent measurement inaccuracies in this mechanical structure which uses springs and potentiometer linkages is obvious when compared to the present availability of solid state type transducers, i.e. thin-film piezoresistive type transducers which are available for measurement of both compression and tensile forces. This is particularly important when the diagnostic test equipment itself has an inherent high accuracy which is then limited to the inaccuracy of such a mechanical type device. Further inaccuracies may result if the operator fails to properly align the base plate such that it rests flat on the brake pedal. If it is skewed the operator may have to apply a greater force to brake the vehicle, resulting in an inaccurate high force reading. In addition, the use of this type of handheld transducer preceded the more recent testing procedures which use handheld keyboard units for communicating with the test stand, such that an operator finds it impossible to perform the brake test procedure with the present type of transducer while at the same time making data entries into the handheld unit. Alternatively, if not concerned with simultaneous brake system measurement and data entry, the present handheld transducer does not allow the operator to keep both hands on the steering wheel, which does not permit simulation of actual use conditions and which may present a safety hazard in the brake test procedure when the automobile is being accelerated on the dynamometer stand.
Another available type of force transducer for measuring brake force, which is not handheld and which is far more accurate than the handheld type device, is a strain gauge sensor. This is typically a piezoresistive type which is provided with a clamp for mounting the strain gauge directly on the surface of the brake pedal. This device, which is semi-stationary, finds utility in developmental type testing where a single automobile is extensively tested for evaluation of new designs, including brake systems, such that the relatively fixed attachment of the sensor to the pedal is acceptable in view of the extended period of test. As may be obvious, however, this type of device does not have practical utility in the production line type of diagnostic testing or in the motor vehicle test procedures, both of which may require the testing of successive automobiles at a rate on the order of two to three minutes each.